Satellite-borne altimeters have had a profound impact on geodesy, geophysics, and physical oceanography. To first order approximation, profiles of sea surface height are equivalent to the geoid and are highly correlated with seafloor topography for wavelengths less than 1000 km. Using all available Geos-3 and Seasat altimeter data, mean sea surfaces and geoid gradient maps have been computed for the Bering Sea and the South Pacific. When enhanced using hill-shading techniques, these images reveal in graphic detail the surface expression of seamounts, ridges, trenches, and fracture zones. Such maps are invaluable in oceanic regions where bathymetric data are sparse. Superimposed on the static geoid topography is dynamic topography due to ocean circulation. Temporal variability of dynamic height due to oceanic eddies can be determined from time series of repeated altimeter profiles. Maps of sea height variability and eddy kinetic energy derived from Geos-3 and Seasat altimetry in some cases represent improvements over those derived from standard oceanographic observations. Measurement of absolute dynamic height imposes stringent requirements on geoid and orbit accuracies, although existing models and data have been used to derive surprisingly realistic global circulation solutions. Further improvement will only be made when advances are made in geoid modeling and precision orbit determination. In contrast, it appears that use of altimeter data to correct satellite orbits will enable observation of basin-scale sea level variations of the type associated with climatic phenomena.

Bathymetry profiles and contour charts have been used to study the distribution of seamounts in the deep ocean basins, but only a small fraction of the seafloor has been sampled by ships. At the present exploration rate it will take several centuries to map significant portions of the seafloor topography. Satellite altimetry, which maps the topography of the equipotential sea surface, is a promising tool for studying the gravity fields of seamounts because all ocean basins can be sampled in a couple of years. Using a model of a Gaussian-shaped seamount loading a thin elastic lithosphere, we develop a new technique for measuring basic characteristics of a seamount from a single satellite altimeter profile. The model predicts that the seamount diameter is equal to the peak-to-trough distance along the vertical deflection profile and that the overall diameter of the signature reveals the age of the lithosphere when the seamount formed. Moreover, the model suggests that these two measurements are relatively insensitive to the cross-track location of the seamount. We confirm these model predictions using Seasat altimeter profiles crossing 14 well surveyed seamounts in the Pacific. We then apply the measurement technique to 26 × 106 million kilometers of Seasat profiles resulting in a new global set of seamount locations. Approximately one quarter of the seamounts identified in Seasat profiles were previously uncharted. Modeling suggests that there is no direct relationship between the size of a seamount and its signature in the geoid; therefore the set of locations is not a straightforward sampling of the total seamount population, but is weighted toward seamounts which are poorly compensated. A preliminary analysis indicates considerable variations in population density and type across the oceans; most notable among them are the absence of seamounts in the Atlantic, variations in population density across large age-offset fracture zones in the Pacific, the prevalence of small signatures in the Indian Ocean, and the existence of linear trends in the large seamounts of the west Pacific.

The Salton Trough represents a complex transition between the spreading center in Baja California and the strike-slip San Andreas fault system and is one of the most active zones of deformation and seismicity in California. We present a high-resolution interseismic velocity field for the Salton Trough derived from 74 continuous GPS sites and 109 benchmarks surveyed in three GPS campaigns during 2008-2009 and previous surveys between 2000 and 2005. We also investigate small-scale deformation by removing the regional velocity field predicted by an elastic block model for Southern California from the observed velocities. We find a total extension rate of 11mm/yr from the Mesquite Basin to the southern edge of the San Andreas Fault, coupled with 15mm/yr of left-lateral shear, the majority of which is concentrated in the southern Salton Sea and Obsidian Buttes and is equivalent to 17mm/yr oriented in the direction of the San Andreas Fault. Differential shear strain is exclusively localized in the Brawley Seismic Zone, and dilatation rate indicates widespread extension throughout the zone. In addition, we infer clockwise rotation of 10 degrees/Ma, consistent with northwestward propagation of the Brawley Seismic Zone over geologic time.